Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sun Oct 16 2022 Valid 12Z Wed Oct 19 2022 - 12Z Sun Oct 23 2022 ...Overview... The medium range period begins Wednesday with an amplified/blocky upper level flow pattern consisting of potent troughing across the eastern half of the U.S. extending south from an anomalously strong upper low in eastern Canada and an upper ridge/high over much of the West. This initial pattern will promote relatively dry weather over much of the lower 48, aside from lake effect/enhanced precipitation (including some snow), along with unseasonably chilly temperatures over the eastern half of the country and very warm weather over the northern half of the West. Then latest guidance still advertises a pronounced pattern change heading into next weekend, with an amplifying upper trough bringing cooler and wetter weather to the West while areas east of the Rockies trend warmer with the greatest anomalies over the Plains. Meanwhile a southern stream Pacific upper low should eventually track toward the western U.S.-Mexico border and may increase rainfall chances over the Southwest to some degree by next weekend. ...Model and Predictability Assessment... The first couple days of the forecast show good agreement and stability overall, as might be expected given the typically good predictability of amplified flow. There are only minor differences with embedded shortwaves and aside from any influence on lake effect precipitation, impacts on sensible weather should be minimal given the otherwise dry pattern. Into day 6 Saturday the guidance is gradually converging with respect to details of the pattern change as an upper trough reaches the Northwest. There were several 00Z GEFS members that depicted a ridge at that time but only one CMC ensemble member (shifted a bit eastward) and no ECMWF members. The 12Z GEFS members show a greater member proportion siding with the majority trough scenario per other operational models and ensemble means, as only one member now shows a ridge. By Sunday there are more differences with trough shape/timing and upstream flow. The 12Z GFS has gone to a closed low in contrast to an open trough in prior runs (and other models) while latest CMC runs are shifted a bit east with the overall Pacific/western U.S. pattern. Elsewhere, guidance continues to waffle for the southern stream Pacific upper low. General trend over the past day has been slower (delaying any moisture reaching the Southwest) but the 06Z GFS took this to an apparent extreme. In contrast the 12Z GFS jumps to the fast side of the spread by Saturday. Finally, models/ensembles remain indecisive about weak Southeast U.S. energy that may separate from the westerlies around the end of the week. The weak and fairly small scale nature of this energy leads to low predictability in the medium range time frame. Overall the ECMWF has been the most consistent with its forecast upper low and track but still shows some run-to-run oscillations (12Z run back to the west). Among ensemble guidance, ECMWF members have a greater percentage depicting such a trough/low relative to the GEFS/CMCens. The relative lack of agreement and low predictability ultimately favor maintaining a conservative approach to the depiction of this feature and its potential effect on the wavy front off the Southeast. The WPC forecast for fronts/pressures was based on a blend of the deterministic 00Z/06Z models to resolve lingering detail questions during the first part of the period, and then incorporated GEFS and EC ensemble means after early Friday as operational models diverged with various details. This approach maintained good continuity with the previous forecast aside from typical nudges reflecting latest trends. ...Sensible Weather and Potential Hazards... The contiguous U.S. should overall be rather dry on Wednesday-Thursday, with the exception of lake effect rain/snow due to upper level energy and the westerly flow across the relatively warm Great Lakes compared to the cold air temperatures. Precipitation amounts are likely to be higher through Wednesday downwind of the Lower Great Lakes in particular but with some lingering showers possible into Thursday. South Florida may see a few rain showers as well. By late in the week through next weekend, the upper trough arrival/amplification across the West and an associated surface front pushing across the region should produce a notable increase in precipitation coverage and intensity. Best focus will be over favored windward terrain over the Pacific Northwest and then extending into the northern-central Rockies, with higher elevation snow and lower elevation rain. Confidence in a meaningful precipitation event is increasing but it will take more time to refine important details that will influence precise timing and totals at any particular location. The southern stream Pacific upper low may reach far enough east by this weekend to produce some rainfall over the Southwest. Meanwhile some rain may be possible along or near the East Coast this weekend, but with low confidence on how much rain could fall onshore. The amplified western ridge in place for the early part of the period will cause warmer than average temperatures across the Northwest in particular, with highs commonly 10-20F above average, potentially leading to some daily record highs through Thursday. Conversely, the anomalous upper low/trough in the East should produce rather cold temperatures more common to November and December from the Plains eastward. Below normal temperatures by 10-25F will be widespread and translate into lows in the 30s spreading south possibly all the way to the Gulf Coast (Mobile, Alabama for example) for Wednesday-Thursday. The Southeast could also see highs in the 50s and 60s, and record low maximum and minimum temperatures are likely to be set in the east-central U.S. As the upper level pattern relaxes around Friday and then transitions to a western trough/eastern ridge pattern, above normal temperatures should get pushed into the central U.S., with the north-central Plains seeing highs of 10-20F above average. Expect temperatures to rebound over the East, reaching near normal over the Southeast after a chilly morning on Saturday while trending above normal over the Northeast. Much of the West should see moderately below normal highs by next Sunday. Rausch/Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml